Proteins in the blood plasma, in the extracellular matrix of the tissues, and in cells are essential for all vital physiological functions. However, many different proteins also contribute to disease. The underlying pathogenetic mechanisms include overproduction of normal proteins with corresponding excessive effects, the production of abnormal proteins with damaging effects, and the incidental subversion of normal protein function leading to damaging effects during intercurrent pathological processes, such as inflammation and microbial infection. There is therefore a need for elimination of a variety of normal or abnormal proteins from the body to provide treatment for many human diseases. Plasma proteins that contribute to pathogenesis of disease include cytokines, lipoproteins, autoantibodies, components of the complement and coagulation pathways, amyloidogenic proteins including monoclonal immunoglobulin light chains, transthyretin, lysozyme and β2-microglobulin, acute phase proteins in particular serum amyloid A protein (SAA), and pentraxins such as serum amyloid P component (SAP). All these different proteins, produced by different cells, are potentially attractive targets for therapeutic elimination in various diseases. However, there are few effective methods for selectively lowering the circulating concentration of specific proteins and those approaches that are available, or have been attempted, are complex, difficult and subject to many extremely challenging problems.
Cytokines are protein hormones produced by and acting on a variety of different cell types, and are critically important mediators of host defence, immunological and inflammatory responses, but their overproduction in some diseases contributes to serious pathology, morbidity and mortality. Antibodies and recombinant binding proteins have lately been used successfully for targeting one particular cytokine, tumor necrosis factor (TNF), and TNF blockade is therapeutically beneficial in rheumatoid arthritis and Crohn's disease, but there are few effective methods for reducing damaging high levels of other cytokines.
Pathogenic overproduction of other normal proteins, such as acute phase plasma proteins, can be reduced by suppressing the activity of an underlying primary disease, but, except in the case of treatable chronic infection, this is usually extremely difficult to achieve. There is no cure for chronic idiopathic inflammatory diseases, such as rheumatoid arthritis or Crohn's disease, or for most malignant neoplasms. Treatment of these diseases and suppression of their activity require regimens including highly toxic anti-inflammatory, cytotoxic and immunosuppressive drugs, and frequently also surgery and/or radiotherapy.
Production of abnormal proteins, whether inherited or acquired as a complication of another primary disease, is also extremely difficult to control. For example, the variant transthyretin and variant fibrinogen molecules responsible for forms of hereditary systemic amyloidosis, can be eliminated from the plasma only by liver transplantation. No such approach is possible with many other hereditary diseases caused by pathogenic variant proteins. Acquired production of abnormal and pathogenic proteins, as in monoclonal gammopathies, can be treated only with powerful cytotoxic drugs or bone marrow transplantation. These procedures incur universal morbidity, and even mortality rates of up 50%, without being uniformly successful.
Another approach has been to remove damaging proteins by extracorporeal absorption, passing blood over more or less selective solid phase media to remove the target protein. This can make a useful contribution to removal of the high concentrations of pro-atherogenic low density lipoprotein in familial hypercholesterolaemia. It has also been attempted, so far unsuccessfully, for removal of two different amyloidogenic proteins: β2-microglobulin in patients on chronic haemodialysis for end stage renal failure, and variant transthyretin in patients with familial amyloid polyneuropathy.
In cases where particular proteins produce their pathogenic effects by binding to specific ligand structures in vivo, a possible approach to therapy is the development of drugs to inhibit such binding. For example, the present inventor has proposed such an approach to the treatment of amyloidosis and Alzheimer's disease, targeting the pathogenic binding of serum amyloid P component to amyloid fibrils by administration of low molecular weight molecules that block such binding (1-5) U.S. Pat. No. 6,126,918). The specific binding of SAP to particular ligands containing anionic groups, including carboxylates and phosphates is known (6-10). In the complex of SAP with deoxyadenosine monophosphate (dAMP), there is a dAMP molecule in the calcium-dependent binding site of each protomer in the homopentameric SAP molecule (10). As a result of base stacking, dependent on hydrogen bonding between pairs of dAMP molecules, pairs of SAP pentamers loaded with dAMP assemble face to face to form a decameric protein-ligand complex (10).
The known low molecular weight ligands of SAP, such as phosphoethanolamine and methyl 4,6-O-(1-carboxyethylidene)-β-D-galactopyranoside (MOβDG), are only bound with modest affinity of about millimolar. This is poor compared to the typical nanomolar affinities of most drug-protein interactions, and suggests that such compounds would be unlikely to be effective as inhibitors of the pathogenic binding of SAP to its ligands in vivo. There remains a need to provide an effective method for the targeted depletion of an unwanted plasma protein population.